Wet forming, sintering behavior, and dielectric properties of BaTi 0.8 Zr 0.2 O 3
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Wet forming, sintering behavior, and dielectric properties of BaTi0.8 Zr0.2 O3 Yoshihiro Hirata and Takahiro Kawazoe Department of Applied Chemistry and Chemical Engineering, Kagoshima University, 1-21-40 Korimoto, Kagoshima 890, Japan (Received 20 September 1995; accepted 10 June 1996)
Density, grain size, lattice parameter, and dielectric properties were measured on BaTi0.8 Zr0.2 O3 sintered at 1100–1600 ±C. The hydrothermally prepared BaTi0.8 Zr0.2 O3 particles of 128 nm diameter were consolidated by filtration of 2-methoxyethanol suspensions and subsequently compressed by isostatic pressing under a pressure of 294 MPa to form a uniform microstructure of high density (52% of the theoretical density). These green compacts were sintered to a relative density of above 99% in the temperature range from 1350 to 1600 ±C where rapid grain growth to above 30 mm occurred. Increase of sintering temperature was accompanied by the increase of lattice parameter and dielectric constant of BaTi0.8 Zr0.2 O3 at room temperature. The sintered BaTi0.8 Zr0.2 O3 showed a diffuse phase transition from paraelectric (higher temperature) to ferroelectric state (lower temperature) at 32 ±C. I. INTRODUCTION
Effects of grain size on dielectric properties of barium titanate (BaTiO3 ) are interesting phenomena from the viewpoints of scientific and technological fields. Previous papers1–4 reported that (i) the dielectric constant of BaTiO3 becomes a maximum near 1 mm grain size and (ii) a phase transition from tetragonal (lower temperature) to cubic form (higher temperature) and broadening of the dielectric constant in the wide temperature range from 240 to 200 ±C occur in the submicrometerrange grains of BaTiO3 . Our previous experiments5,6 on the processing and measurement of electrical properties of BaTiO3 using a nanometer-sized fine powder also provided the information described above. In this study, we investigated the grain size effects on crystal structure and dielectric properties of barium zirconium titanate with a composition of BaTi0.8 Zr0.2 O3 . Substitution of Zr for Ti in BaTiO3 is accompanied with the decrease of transition temperature between tetragonal and cubic forms, leading to the increase of dielectric constant at room temperature.4,7 To achieve a dense BaTi0.8 Zr0.2 O3 solid solution with a fine-grained microstructure, we used a wet forming process of a fine BaTi0.8 Zr0.2 O3 powder prepared by a hydrothermal process. The green compacts were sintered in the temperature range from 1100 to 1600 ±C to study the densification behavior and grain growth of the nanometer-sized BaTi0.8 Zr0.2 O3 particles. The dielectric properties were measured at 230 to 210 ±C to evaluate them in relation to the crystal structure and microstructure. II. EXPERIMENTAL PROCEDURE
A high purity BaTi0.8 Zr0.2 O3 powder with an average diameter of 128 nm (particle size distribution of 60 J. Mater. Res., Vol. 11, No. 12, Dec 1996
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to 210 nm), supplie
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